A side sill which is provided in a vehicle body lateral side portion functions as a structural component in the longitudinal direction of the vehicle. When the front surface of a vehicle is in a collision, the side sill receives in a favorable buckling direction strength-wise a portion of the load which is applied from the front, and provides resistance to the collision. Accordingly, normally, a side sill has a closed cross-sectional structure extending in the longitudinal direction of the vehicle, in which an outer side panel and an inner side panel are bonded together via flanges which are provided on top and bottom edges thereof, and which provides excellent strength and rigidity. However, in recent years, even greater improvements in safety have been demanded during a side-on vehicle collision, and the demands have included making improvements to strength and rigidity during a collision from the side surface of a side sill.
For example, one technology employs a structure in which a rocker reinforcement is formed in an arch shape and which provides reinforcement against vehicle side surface collisions (see Patent document 1).    Patent document 1: Japanese Patent Publication No. 2990983